By Thursday, there were six deaths reported in Illinois and 18 deaths reported in Massachusetts, and 41 states reported high levels of illness.

Hardest hit: the East, South and Midwest, said Michael Jhung with the Center for Disease Control and Prevention's influenza division. The West, Southwest and some parts of Maine and Wisconsin have had very little flu.

That's small consolation to the miserable. For those who do fall ill, the flu is a stomach-churning, head-burning, cough-wracked, muscle-aching experience. It's particularly dangerous for the elderly, young children and people who have compromised immunity because of other illnesses or chronic conditions such as diabetes or cancer.

The 2013 season is particularly wretched because one of the major strains in this year's flu mix hasn't been seen for five to nine years and people's antibodies for it have waned, said Arnold Monto, a professor of epidemiology and member of the Infectious Diseases Society of America's Influenza Advisory Group, which works with the CDC.

There are three types of flu circulating in the USA: H3N2, H1N1 and Influenza B. H3N2 is by far the most common and most likely to put people in the hospital with complications. All three are included in this year's flu vaccine.

For those who haven't been vaccinated, the chances are they don't have antibodies for H3N2. It's been nine years since the 2002-2003 flu season, when H3N2 was in high circulation in the country. That season, about 95% of the flu was made up of H3N2. In 2007-2008, it was 75%, the CDC's Jhung says. Since then, other strains have been more common, so resistance to the strain has ebbed. The virus has also mutated.

"Flu viruses change all the time. They change in big ways, which gives them a different H and N designation, and they change in small ways. So last year's H3N2 can be a little bit different from this year's H3N2 virus. If you've been exposed to a similar virus, you'll have some protection, but not full protection," Jhung said.

Boston had declared a public health emergency. The city has had more than 700 cases of flu - "the worst season we've seen since 2009," according to Mayor Thomas Menino. The city plans a free vaccination campaign this weekend in an effort to slow the virus spread. The mayor pleaded with people, "If you're sick, please stay home from work or school."

Each year, vaccine manufacturers make an educated guess about the strains of flu likely to circulate worldwide. CDC Director Tom Frieden says that even if the vaccine is not perfect, "it is, by far, the best tool we have to prevent influenza, which remains a serious and potentially fatal disease."

At Boston's Beth Israel Deaconess Medical Center, Jim Heffernan, chief of primary care, faced an overflowing emergency room without "enough places to put people. It just snowballs." The hospital hotline rang ceaselessly, and Beth Israel spokeswoman spokeswoman Kelly Lawman said, "We had to open a new unit to accommodate all the patients."

The emergency room at Ohio State University's Wexner Medical Center in Columbus developed a fast-track system to move college students with the flu quickly through the emergency room to keep beds free for more vulnerable patients. It directed others to urgent care centers and their family physicians. "It's tough when the hospital is totally full and there's nowhere to put patients," said Mark Moseley, Wexner's assistant chief operating officer. "For good or ill, society perceives the emergency room as the place to go when you have a cold or the flu."

In Cleveland, a flu task force meets for 20 minutes every morning to handle the crisis at the University Hospitals Case Medical Center. "This is a really extreme challenge to the system," says Michael Anderson, chief medical officer at University Hospitals, who leads the 20-person meeting with doctors from regional hospitals, nursing directors, pharmacists, ambulance supervisors and others.

The medical system's senior leaders discuss hospital bed capacity, the health of its staff, where to shift patients in the regional hospital system and supplies of crucial items such as Tamiflu and face masks. Thursday morning, the task force decided to limit patient visitation, keeping away from the hospital any visitors who have flu-like symptoms, Anderson said. Computer systems report a census of hospital beds hourly and manage patient surges by directing ambulances and physician referrals to hospitals with capacity.

Around the country, the challenges mount:

•Rhode Island's health director Michael Fine said the state has had 380 flu hospitalizations this season compared with 90 for all of flu season last winter. Only 40% of the state has been vaccinated, Fine said, and he urged people to step up and take care of this quickly.
•At many Milwaukee-area hospitals, emergency rooms were so overwhelmed with high demand Wednesday that five to eight of the county's 11 hospitals were forced to temporarily, periodically shut down emergency departments except for life-threatening cases.
•In Springfield, Ore., more than one in four of the 138 residents and 57 staffers at an assisted living facility reported flu symptoms, and 13 people were hospitalized, Lane County chief health officer Patrick Luedtke told the CDC. Jessica Knox, executive director at Emeritus at Springfield-The Briarwood, told The Register-Guard newspaper that the on-site clinic offered vaccines to residents, but many had elected not to get a flu shot.
•Springfield, Mo., has seen 500 cases in the past five weeks. That's more than double the rate of sickness it saw during last year's flu season when there were 500 cases in 12 weeks, according to Mike Brothers, spokesman for the Springfield-Greene County Health Department.
•In Chicago, Northwestern Memorial Hospital was one of eight hospitals that put out word they could not accept ambulances because their emergency rooms were inundated with flu patients and operating at capacity, said Rahul Khare, an emergency physician. Normally, no more than two hospitals in the area night be in that condition, he said.
•In Salisbury Township, Pa., Lehigh Valley Hospital-Cedar Crest normally admits 10 to 20 patients with influenza-like illness in a week at this time of year. After admitting more than 100 from Dec. 31 to Jan. 4, the hospital erected a tent. At the surge tent, as it is called, flu patients who are not sick enough to be admitted can be treated more efficiently and quickly, said Terry Burger, a nurse who is director of infection control for the hospital's parent, the Lehigh Valley Health Network. "People with a milder illness ... can be seen in a surge tent relatively quickly and then discharged."

The CDC says it's too early to estimate how many billions of dollars this year's flu will cost the U.S. economy.

The most recent CDC study , published in 2007 and based on a 2003 population, put the direct medical cost at an average of $10.4 billion and projected lost earnings at $16.3 billion. The total estimated economic burden, including the lost lifetime earnings of people who die from the flu, hit $87 billion, the study said.

At $87 billion, it was 0.79% of the USA's 2003 gross domestic product, the study says.

Since then, many more people are being vaccinated each year against the flu, which could reduce the economic impact, says CDC health economist Martin Meltzer.

Another CDC study, published last year, found that parents of flu-stricken children younger than 5 had medical expenses ranging from less than $300 to about $4,000, and missed 11 to 73 hours of work, depending on whether their child was hospitalized. Those estimates were based on 2009 costs.